There is an increasing interest in PEVs (plug-in electric vehicles), particularly with the introduction in 2011 of the Nissan Leaf and the Chevrolet Volt. The Nissan Leaf is a purely electric car that requires no gasoline and provides the consumer with a 100-mile driving range per charge. The Chevrolet Volt is a PHEV (plug-in hybrid electric vehicle) which extends its electric driving range with a gasoline generator. Owners can charge the Nissan Leaf and the Chevy Volt using either a 240V or 120V power supply. Charging at 240V requires a separate infrastructure (EVSE) that supplies electric energy for charging electric vehicles and plug-in hybrids, while 120V charging can be done from a regular house outlet.
Though the chargers are similar for all EVs, there are several differences between the battery sizes of various EVs. For example, the Leaf has a battery size of 24 kWh while the Volt battery is at 16 kWh. Fully charging a Leaf at 240V/16 A requires around 6.5 hours while charging at 3.7 kW, whereas fully charging a Volt at 240V/16 amps requires around 4 hours while charging at 3.7 kW. On the other hand, charging at 120V takes substantially, longer, as approximately 20 hours are required to fully charge the Leaf at 1.4 kW, while approximately 11.5 hours are requires to completely charge the Volt at 1.4 kW.
On average, a typical household draws 0.7 kWh of load from their local power utility. This might consist of typical household appliances such as washers, dryers, ovens, etc. However, the household load can change depending on several variables, notably weather related HVAC usage. There may also be luxury variables such as pools, spas and on-site PV (photovoltaic) generation. In contrast, an EV can draw up to 3.7 kW per hour. This presents a problem because one EV owner alone can indirectly add 4 households worth of load to a service transformer, which can place that transformer at risk of being seriously overloaded. Modern transformers can withstand a certain degree of overloading, however as the popularity and adoption of PEVs grows, the risk of multiple PEVs clustered on a single transformer will increase that risk. Moreover, the next generation of PEVs are poised to come out with bigger batteries that require higher kWh charging, further heightening the risk to, transformers.
When buying a PEV, the consumer has no obligation to inform the utility company of their purchase. As a result, some utility companies have initiated programs where customers can work with the utility to benefit from special power pricing structures, and utilities can be made aware of the additional load to their grid. For example, San Diego Gas and Electric (SDG&E) has participated in the federally funded EV Project; this project works with different utility companies to help provide new PEV customers with free 240V chargers. In addition, SDG&E has put most of these customers on special billing rates that incentivize owners to charge during off-peak hours. Knowing the location of the PEVs, SDG&E is then able to monitor load on the infrastructure that serves that customer. This situational awareness allows the utility to limit power interruptions and to preserve power quality. Additionally, knowing the location of electric vehicles facilitates programs such as demand response to shut off charging when the grid is under stress, and Smart Grid applications such as Vehicle-to-Grid distributed energy storage. As incentive programs such as these come to an end, it is increasingly difficult for utilities to keep track of the influx of PEVs to their grid. As such, it is incumbent upon the utilities to employ advanced methods of tracking electric vehicle charging.